1. Field of the Invention
The present invention relates to a display driver circuit and a method for driving a display display device such as a current-drive type of light-emitting device such as an organic electro luminescence (EL) device.
2. Description of the Background Art
Currently, liquid crystal display (LCD) devices have been extensively used as light, thin, low power consumption display device. To fully disseminate as successive generation of the LCD, research and development have been made on display devices containing self-luminous devices arranged in a matrix form, such as organic EL devices, inorganic EL devices or light emitting diodes (LEDs). A display device including the self-luminous devices has its response speed more improved than an LCD device and its viewing characteristics not affected by viewing angle, thereby having the possibility of attaining its improvable viewing intensity, contrast and definition. In addition, unlike the LCD device, the self-luminous display device requires no backlight, so that the device can be made thinner and lighter and can contribute further low power consumption.
In order to display gradation on a self-luminous display device, the display device has its display screen formed by a lot of pixels, for each of which a light-emitting device and switching devices for driving the light-emitting device have to be provided. Generally, a light-emitting drive circuit, of which detailed description will be made later, comprises a switch transistor for causing, when enabled by the associated selector line, the associated capacitor to hold display data, or a gradation signal, fed from the associated data line, and a drive transistor for supplying the light-emitting device with current according to the gradation signal held in the capacitor. To these transistors, applicable are single-channel amorphous silicon thin-film transistors which may be manufactured in a simpler process and a reuniform in operational properties. The thin-film transistor is, however, notorious for its threshold voltage (VT) typically variable, or set off, due to its drive history, i.e. VT shift being notably caused.
If the threshold voltage of the drive transistor varies, the currents supplied to the light-emitting devices do not correspond to data to be displayed, so that they cannot appropriately emit light with gradation in luminance. The threshold voltage of the light-emitting devices is dependent upon the history of light-emission thereof, i.e. the history of the drive transistors driven in the past, thus resulting in fluctuation in light-emission properties of the display devices to deteriorate the quality of an displayed image.
In order to solve those problems, Japanese patent laid-open publication No. 2006-301250 discloses a display driver, in which the threshold voltage of a drive transistor of each light-emitting drive circuit is measured prior to displaying an image to produce compensation data for each pixel based on the measurement results, and then a voltage corresponding to data to be displayed is added to a voltage corresponding to the compensation data, thereby correcting a driving voltage to feed it to the light-emitting drive circuit.
This conventional display driver is adapted to measure, in advance of the drive operation for displaying, the threshold voltage of the light-emitting drive transistor of each display pixel on the display screen to store the measurement result as threshold compensation data, and then, during a drive operation for displaying, produce a compensation voltage from the threshold compensation data to correct the gradation effective voltage representative of display data externally provided with the compensation voltage, thus adaptively controlling the current passing through the drive transistor for driving the light-emitting device. The conventional display driver thus can compensate the fluctuation, or offset, in threshold voltage due to the drive history of the drive transistor so as not to cause deterioration in display image quality.
The conventional display driver, however, requires a digital-to-analog converter for generating a compensation voltage for compensating for fluctuation in voltage threshold of the light-emitting drive transistor as well as an analog-to-digital converter for producing a threshold detection voltage for use in measuring the threshold voltage. It therefore poses a problem of an increase in circuit size, thus leading to an increase in area for laying out circuit elements.